The present invention relates to thermoplastic structures which exhibit high bending stiffness. These structures exhibit a paper-like feel and can be used to replace paper in numerous applications, such as various printing and imaging applications.
Cellulose fiber, i.e. paper, is typically used as a substrate for imaging technologies, such as for photographs, ink jet images or digital images. Advantageously, paper has good stiffness properties and is low in cost. However, paper as an image-receiving substrate poses several difficulties in its manufacturing process and its use.
For example, a significant weakness of paper is moisture uptake. Paper must be specifically and expensively formulated to resist moisture absorption. For example, in the formation of color paper for photographs and other imaging technologies, paper requires application of a polymer layer, typically a polyethylene layer. In addition to waterproofing the paper, this layer provides a smooth surface upon which imaging layers are formed. However, the formation of such a suitably smooth surface is difficult requiring great care and expense to ensure proper laydown and cooling of the polyethylene layers. An air bubble may become trapped between the forming roller and the polyethylene, forming a pit that will cause defects in the imaging surface.
Moreover, the assembly process of applying a polymer surface to a paper base involves at least a two step operation. The paper base is made and then transferred to an extrusion machine for application of the polymer layer. The machine that produces the raw paper base is specialized for such purpose and often does not efficiently run the same speed and width as machinery which applies the polymer layer.
Additionally, substrates which contain paper have a tendency to curl during development and storage. Such curl is thought to be caused by the disparate properties of the layers making up the substrate as it is subjected to the developing and drying processes. This is especially true when the paper at the middle of a imaging substrate gains or loses water, thereby pushing against stiffer plastic moisture insensitive layers bonded to the outside of the paper. Also, substrates containing paper will curl when they are subjected to extended high humidity or extremely low humidity.
Also, developing and processing images using substrates containing paper present difficulties. Substrates with a paper base do not have the required strength properties to be reliability processed in wet chemistry common with the imaging development process. For example, when the backlighted photographic display materials are processed, the web can break causing a loss in efficiency in commercial photoprocessing labs. Further, thin papers (e.g., as disclosed in U.S. Pat. No. 5,212,053) are not strong enough for efficient transport in digital printing equipment such as ink jet printers or thermal dye transfer printers.
Paper-like films (that is, films that exhibit “paper-like” surface textures) which do not contain paper fiber as a base have been disclosed. (See e.g., PCT/US95/11222: PCT/CA93/00385; and EP 91307049.6.) These paper-like films can be fabricated to be heavy or lightweight. However, both types of these films present limitations. For example, these lightweight films are not stiff like paper, and thus are not desirable for commercial use, such as for photographic imaging technologies. The heavy films are stiff. However, since these heavy films are dense solid structures, or are filled with certain additives, they have a low yield. That is, for a fixed weight of production material, the area of usable film product is low vis-à-vis less dense structures. Thus, the production of these heavy films is not cost efficient.
Accordingly, there remains a need for paper-like image-receiving substrates that are lightweight yet stiff enough for use in commercial applications. Additionally, there remains a need for paper-like image-receiving substrates which have the desired stiffness properties but minimize the material used in the product, thereby reducing cost. Also, there remains a need for paper-like image-receiving substrates which are easier to manufacture.